Even though the bitcoin network itself handles fundamental transaction processing, web shopping carts will need additional amounts of payment processing to match purchases to incoming payments, be notified when a payment is processed (currently bitcoin purchase can take multiple hours to be confirmed), though order fulfillment and completion.

Even a simple "donate" button will require some sort of payment processing gadgetry a la Paypal's web button engine, if you want people to provide names and/or messages along with their donation.